Sun VirtualBox or VMWare for a virtual ISPConfig?

Here is what I'd like to achieve:
A VirtualMachine with Ubuntu 8.04, running ISPConfig(v3 when it's ready)Why:
Need to be able to move hardware frequently, but wanna run a steady and well tuned system.

This extends also to the question whether it'll be Linux or Windows based?
In the past two years I had to setup our Linux-Server and ISPConfig seven times, due to ISPs issues!! On the other hand we had another very reliable ISP providing a Windows-Server… so I could maybe just run one Server… as neither of the two physical machines have a big load at all.

So far I got some experience with VMware-Server and the VMware-Player on the Windows side, using them in a teaching environment with LAN access only. The Server seemed rather slow, so we used the Player once the VMs were built — no experience with VMware-ESX.

Questions:
Windows-Side:

A Windows-Server 08 with 2GB RAM should be sufficient, right?

How does it work when your ISP gives you only one IP and the Windows-Server already has the IIS running, providing needed HTTP & FTP?(Email, some more websites, MySQL etc. will run in the virtual sys)

Would an Image with Ubuntu 8.04+ISPConfig set to ~1GB RAM run well in the VMware-Player(neither of the two have high loads and I ran Ubuntu 6.06+ISPConfig on 512RAM just fine)

Any good reasons for using Sun-VirtualBox — I tried 2.0.2 and it crashed a lot…!?

Linux-Side:
(Here the base would be an Ubuntu 8.04 Server(1.5GB), running a virtual Ubuntu 8.04+ISPConfig set to ~1GB RAM or more!?)

How is VWware on the Linux-Side? I've never tried.

Would the Sun-VirtualBox (headless) be the better choice? Strong points?

I'm mostly seeking information and experience on how this works and behaves in a production environment, as I'll step out of our tiny LAN…

Correct me if I'm wrong Till, but with openvz the guest is not independent from the host, as openvz is a paravirtualization based on the host being used.
I really want a VirtualMachine so I don' depend on the host system and can move it freely. Speed issues are less important for me, too.

OpenVZ is paravirtualization, but this does not mean that you can not move it freely. As long as you dont want to move it from Linux to Windows, you can move it around between linux servers with different hardware and different Linux distributions etc.

Somehow I'm still thinking I'd rather wanna have something I can switch from Linux to Windows. The WinServer is run on a 2.4G DualCore Sys with 2G RAM that's mostly in idle, as the stuff running on port 80 ist pretty much used only by staff out of the office…
And the Linux Sys is an old 2G SingleCore with 1G RAM, with an average load of 0.2-0.4, running emails mostly.
So, anyone done this and can tell why/why not?